In an automobile, it has been proposed to allow a rear seat to be sprung up in a lateral direction so as to make the space inside a vehicle compartment large. In the case where such a rear seat is sprung up in a lateral direction (in the case of lateral rotation in a raising direction), even if a slightly large force is applied, the self weight of the rear seat acts, so that the problem of a violent collision against a stopper and the like does not occur so much. However, in the case where the rear seat is returned to its original state (in the case of lateral rotation in a lowering direction), the rear seat is suddenly lowered in the lateral direction due to the effect of the self weight of the rear seat and violently collides against the stopper and the like, generating a large unpleasant noise. To allow the lateral rotation in the lowering direction of this rear seat to be effected slowly, it is sufficient to use a damper for absorbing the lateral rotational energy. As such a damper, one using a fluid is known, and with this damper using the fluid, the damper must inevitably become large-sized in order to obtain a large damping force, and a large-scale seal is required to prevent the leakage of the fluid. Particularly in the case of a damper for an automobile seat mechanism in which a backrest is arranged to be extended with respect to the rear seat by rotating the backrest in a backward direction with respect to the rear seat, and the rear seat is subsequently arranged to be sprung up in the lateral direction so as to secure an accommodating space inside the vehicle compartment, it is necessary to damp large rotational energy based on the total load of the backrest and the rear seat. Therefore, with a compact damper which merely makes use of a fluid, it is difficult to obtain the targeted large attenuation.